FIG. 10 shows a driving force transmission mechanism which is carried on an automobile or the like and which is used to transmit the driving force from an internal combustion engine to axles. In the driving force transmission mechanism, outer ring members 1, 1 for a barfield-type constant velocity joint and outer ring members 2, 2 for a tripod-type constant velocity joint are connected to one another by spline shafts 3, 3. A differential gearing 4 is interposed between the outer ring members 2, 2 for the tripod-type constant velocity joint. Both of the outer ring members 2, 2 for the tripod-type constant velocity joint are arranged near the differential gearing 4. On the other hand, the outer ring members 1, 1 for the barfield-type constant velocity joint function to transmit the rotary driving force to unillustrated wheels. In FIG. 10, reference numeral 5 indicates a half shaft for bridging the differential gearing 4 and the outer ring member 1 for the tripod-type constant velocity joint.
The end of the half shaft 5 is connected to the outer ring member 1 for the barfield-type constant velocity joint by a plurality of rollable balls 6.
FIG. 11 shows a schematic perspective view illustrating the entire outer ring member 1 for the barfield-type constant velocity joint, and FIG. 12 shows, with partial cutaway, a sectional view illustrating the outer ring member 1 for the barfield-type constant velocity joint. The outer ring member 1 for the barfield-type constant velocity joint is made of carbon steel, and it has a shaft section 7 and a cup section 8 which are integrally formed.
In particular, six ball-rolling grooves 9a to 9f are formed on the inner wall surface of the cup section 8 so that the ball-rolling grooves 9a to 9f are spaced from each other by predetermined angles in the circumferential direction (see FIG. 11). The ball-rolling grooves 9a to 9f are provided for rolling the balls 6 (see FIG. 10). The ball-rolling grooves 9a to 9f are provided to extend to the vicinity of the end of the cup section 8 along the inner wall surface of the outer ring member 1 for the barfield-type constant velocity joint (see FIGS. 11 and 12). On the other hand, a center hole 10 is provided at the end of the shaft section 7 (see FIG. 12).
The outer ring member 1 for the barfield-type constant velocity joint is manufactured by cold forging. At first, as shown in FIG. 13A, a pretreatment is applied to a workpiece 11 of a columnar material having a diameter slightly larger than that of the shaft section 7. That is, the workpiece 11 made of carbon steel is subjected to a spheroidizing annealing treatment for depositing cementite in a spherical form in a metal microstructure. Subsequently, a lubricating chemical conversion coating is formed on the surface thereof by bonderizing. In general cold forging, a coating of zinc phosphate is frequently used as the lubricating chemical conversion coating.
Subsequently, an unillustrated first forging die is used for a primary forging (forward extrusion) to the workpiece 11 on which the lubricating chemical conversion coating has been formed. That is, one end surface of the workpiece 11 is pressed against a cavity which is formed in the first forging die and which has a diameter smaller than that of the workpiece 11. Accordingly, the other end surface of the workpiece 11 is forcibly inserted into the cavity. As a result, as shown in FIG. 13B, a primary forged product 13 is obtained with the shaft section 7 and a diametrally reduced section 12 having a reduced diameter in a tapered form. The shaft section 7 and a diametrally reduced section 12 are formed by the other end of the workpiece 11.
Subsequently, a secondary forging (upsetting forming) is performed for the primary forged product 13. Specifically, an unillustrated second forging die is used to successively compress only a large diameter section 14 of the primary forged product 13 as shown in FIGS. 13C and 13D so that the large diameter section 14 is diametrally expanded to obtain a secondary forged product 15.
The secondary forged product 15 is subjected to a low temperature annealing treatment for removing any stress or the like and a shot blast treatment for removing any oxided scale or the like generated during the low temperature annealing treatment. Further, bonderizing is performed to form a lubricating chemical conversion coating of zinc phosphate or the like on the outer surface of the second forged product 15.
Subsequently, a tertiary forging process (backward extrusion) is applied to the secondary forged product 15 which is arranged in a cavity of an unillustrated third forging die after the respective treatments as described above. The diametrally expanded large diameter section 14 is elongated, ball-rolling grooves 17a to 17f are formed on the large diameter section 14, and the cup section 8 is formed.
That is, an unillustrated punch, which has a projection to form the ball-rolling grooves 17a to 17f, abuts against a central portion of one end surface of the-cup section 8. Subsequently, the end of the shaft section 7 is pressed to displace the secondary forged product 15 toward the punch. Accordingly, the secondary forged product 15 is crushed by the punch while the large diameter section 14 is surrounded by the inner wall of the cavity. Consequently, the large diameter section 14 is elongated, and the ball-rolling grooves 17a to 17f, which have shapes corresponding to the shape of the projection of the punch, are formed on the large diameter section 14. Thus, a tertiary forged product 18 is obtained as shown in FIG. 13E.
A low temperature annealing treatment is applied to the tertiary forged product 18 to soften the tertiary forged product 18. After that, a shot blast treatment is performed again, and a lubricating chemical conversion coating is formed again by the bonderizing treatment as described above. When various treatments are performed as described above, it is possible to avoid cracks which would be otherwise caused by the tensile stress on the inner surface of the cup section 8 when ironing is performed in the next step.
Finally, an unillustrated fourth forging die is used to apply the ironing (final sizing forming), i.e., a quaternary forging process for finishing the final product shape. Accordingly, the outer ring member 1 for the barfield-type constant velocity joint as the finished product is consequently obtained (see FIG. 13F).
As clearly understood from the above, in the conventional production method, it is necessary that various treatment operations, which are complicated and require a long period of time, are successively performed before forging and processing the workpiece 11, the secondary forged product 15, and the tertiary forged product 18. For this reason, the entire time required for the production is prolonged until the outer ring member 1 for the barfield-type constant velocity joint is obtained. In other words, in the conventional production method, it is impossible to efficiently manufacture the outer ring member 1 for the barfield-type constant velocity joint.
Further, it is inevitably difficult to inexpensively supply the outer ring member 1 for the barfield-type constant velocity joint under the situation since it is impossible to mass-produce the outer ring member 1 for the barfield-type constant velocity joint as described above.
An object of the present invention is to provide an outer ring member for a constant velocity joint and a method of manufacturing the member in which the outer ring member for the constant velocity joint can be efficiently manufactured by performing continuous forging machining without various treatment operations such as low temperature annealing and bonderizing treatment, and it is possible to reduce production cost.